The present invention relates to the manufacture of integrated circuits, and more specifically to a method for depositing a nitrogen-doped FSG film for use in such circuits. Films deposited according to the present invention are useful in various applications, and are particularly useful in the formation of intermetal dielectric layers and in copper damascene applications.
In conventional integrated circuit fabrication, circuit elements are formed by etching a pattern of gaps in a layer of metal, which are then filled with a dielectric. As efforts continue to include ever greater levels of integration on semiconductor chips, there has developed a persistent need to make circuit components (such as transistors, capacitors, etc.), smaller, bringing the components closer together, thereby allowing a greater number of components per unit of chip area. Increasing the component density on semiconductor chips results in increased sensitivity of operating speed and power consumption on the dielectric constant k of the material used to insulate the electrically conductive structures. If the dielectric constant is too high, the capacitance between the chip""s metal lines becomes too large, creating undesirable cross talk across layers.
Various forms of silicon oxide or silicon-oxide-based glass are commonly used as the insulating material in integrated-circuit fabrication. While silicon oxide has an acceptably low dielectric constant for many applications, a lower dielectric constant is preferable for some applications, such as those involving a high density of circuit components. The lower dielectric constant reduces RC time delays, contributing to an overall improvement in the circuit""s operation speed. One method of forming an insulator with a lower dielectric constant than undoped silicate glass (xe2x80x9cUSGxe2x80x9d) involves adding fluorine to silicon oxide during a chemical-vapor-deposition (xe2x80x9cCVDxe2x80x9d) process. The presence of the fluorine dopants in the resulting fluorinated silicate glass (xe2x80x9cFSGxe2x80x9d) is known to have the desired lowering effect on dielectric constant.
Another factor to be considered in developing methods for depositing films with appropriate dielectric constant is that copper, which has lower resistance than conventional aluminum alloys, is poised to take over as the main on-chip conductors for all types of integrated circuits. It is more difficult to etch copper than aluminum and a specialized process, referred to as a xe2x80x9cdamascene process,xe2x80x9d has therefore been developed for the fabrication of copper-based integrated circuits. Thus, in damascene processes, dielectric layers are first deposited as an integrated stack, which is then etched to form gaps to be subsequently filled with the conductive material. A barrier layer, which can be overlying or underlying, is commonly included to prevent diffusion of copper into adjacent dielectric layers. Some integrated stacks used in damascene processes also use a layer known as an xe2x80x9cetch stopxe2x80x9d or xe2x80x9chardmaskxe2x80x9d to provide for selective etching of the film. Silicon nitride (SixNy) is a material commonly used for such applications, for example when forming vias between layers containing metal lines.
Deposition of USG and FSG films for both gap-fill and damascene applications has previously been undertaken in high-density plasma (xe2x80x9cHDPxe2x80x9d) CVD systems. In such systems, deposition is accomplished by forming a plasma in a chamber from a mixture of gases containing the necessary elemental constituents of the desired film. In the case of gap-fill applications, the wafer may be processed in the plasma while a bias is applied to the wafer. The bias accelerates ions from the plasma to the wafer so that the wafer is bombardedxe2x80x94material that might prematurely close the gap is sputtered away while material from the plasma simultaneously deposits to fill the gap. The FSG gap-fill process is a generally good process scheme in terms of reliability, stability and throughput. HDP-FSG films deposited in gap-fill applications typically have a fluorine concentration of about 5.5-7.0 atomic percent (at.%) and a dielectric constant k of about 3.7, compared to a value of k about 4.0 to 4.3 for conventional undoped silicon oxides.
While the use of FSG has provided an insulating material with a reduced dielectric constant compared to USG, further reductions remain desirable because such decreases translate directly into increased operation speed and circuit performance. It is further desirable to be able to deposit a film with the desired decreased dielectric constant while simultaneously achieving greater stability for the film.
Embodiments of the present invention provide such a silicate-glass-based insulator having both a lower dielectric constant and improved stability. By including a nitrogen-containing gas in the mixture that is supplied to the deposition chamber (in addition to the gases otherwise used to produce FSG), a nitrofluorinated silicate glass (xe2x80x9cNFSGxe2x80x9d) film can be deposited on a substrate. Such an NFSG film has a dielectric constant approximately 5% lower than the dielectric constant of an FSG layer deposited without using a nitrogen-containing gas, but under otherwise similar conditions. This reduction in dielectric constant, attributable to the inclusion of nitrogen dopants in the film, permits increased device speed, and the enhanced stability exhibited by the film lessens integration concerns that otherwise exist with both FSG and USG. The NFSG layer also exhibits excellent adhesion to an overlying or underlying barrier layer as may be required in certain embodiments. Various embodiments of the invention are applicable both to damascene and gap-fill applications. The gap-fill capability of an NFSG layer is also improved over FSG or USG films deposited under otherwise similar conditions.
In one embodiment that is amenable to gap-fill applications, a method is provided for depositing an NFSG film on a substrate in which a gaseous mixture of silicon-containing, fluorine-containing, oxygen-containing, and nitrogen-containing gases is provided to a chamber. A high-density plasma is generated from the gaseous mixture, where xe2x80x9chigh-densityxe2x80x9d is understood in this context to mean having an ion density that is equal to or exceeds 1011 ions/cm3. A bias is applied to the substrate at a bias power density between 4.8 and 11.2 W/cm2, and the NFSG layer is deposited onto the substrate using the plasma. In one particular embodiment, the bias power density is 8.3 W/cm2. In a preferred embodiment, the nitrogen-containing gas is N2, but may be a different nitrogen-containing gas such as N2O, NH3, or NF3. The fluorine-containing gas is preferably SiF4 and the silicon-containing gas is preferably a silane. The ratio of flow rate for the oxygen-containing gas to the combined flow rate for all silicon-containing gases in the gaseous mixture should be between 1.0 and 1.8, and preferably within the range of 1.2-1.4. The N2 flow rate should be in the range 10-60 sccm, preferably 20-40 sccm, which may be adjusted as appropriate for alternative nitrogen-containing gases according to their stoichiometry. Using optimized parameters results in deposition of an NFSG film with a lower dielectric constant and better adhesion properties than FSG. The method provides a gap-fill capability that can substantially fill a gap with an aspect ratio greater than 3.2:1.
In another embodiment that is amenable to damascene applications, a method is provided for depositing an NFSG film on a substrate by providing a gaseous mixture of silicon-, fluorine-, oxygen-, and nitrogen-containing gases to a chamber, from which a high-density plasma is generated. A bias with a power density between 0.0 and 3.2 W/cm2 is applied to the substrate and the NFSG layer is deposited using the plasma. For damascene applications, the bias power density is preferably 1.6 W/cm2, and the ratio of flow rates of oxygen-containing to all silicon-containing gases in the gaseous mixture is between 1.2 and 3.0, with a preferred range of 1.8-2.5. As for the embodiments amenable to gap-fill applications, it is preferred that the nitrogen-containing gas be N2, although other gases such as N2O, NH3, or NF3 may also be used; the preferred fluorine-containing gas is SiF4, and the preferred silicon-containing gas is a silane. The N2 flow rate is preferably in the range 30-120 sccm, although this rate may be adjusted when using alternative nitrogen-containing gases according to their stoichiometry. In related embodiments, the NFSG layer is deposited on a barrier layer previously formed on the substrate; the barrier layer is preferably a silicon nitride layer. Where the NFSG layer is deposited as part of a damascene process, it is preferred that the substrate be heated by an in situ plasma that does not contain oxygen prior to depositing the NFSG layer. As for the gap-fill applications, use of optimized parameters permits deposition of an NFSG film with a lower dielectric constant and better adhesion properties than FSG.
The methods of the present invention may be embodied in a computer-readable storage medium having a computer-readable program embodied therein for directing operation of substrate processing system. Such a system may include a process chamber, a plasma generation system, a substrate holder, a gas delivery system, and a system controller. The computer-readable-program includes instructions for operating the substrate processing system to form a thin film on a substrate disposed in the processing chamber in accordance with the embodiments described above.